New to Ubuntu 12.04 (used since 8th April). I have recently been using Firefox for an online course and while working on this virtual PC site, the screen dims and I am frozen out of interacting with my desktop. (I cannot switch to any other tabs in Firefox). Power setting set to suspend after 30 mins. The desktop is connected to the mains. Moving the cursor around appears to have no effect. This is making the course difficult to follow so any ideas would be appreciated. Thanks